1. Field of the Invention
The present invention relates to an ink set for ink-jet recording having a pigment-based ink and a dye-based ink, a method for producing the same, and an ink-jet printer.
2. Description of the Related Art
Dye-based inks, which are excellent in handling performance, which hardly deposit precipitates, which exhibit satisfactory color vividness or brightness, and which contain various types of anionic dyes as coloring agents, have been hitherto widely used as inks to be used for the ink-jet recording. On the other hand, pigment-based inks, which are slightly inferior to the dye-based ink in relation to the color vividness but which exhibit high image densities and low blurring properties, are also used as inks for ink-jet recording. Those having been used as the pigment employed as the coloring agent for the pigment-based ink as described above include negatively chargeable and self-dispersible pigments which do not require, in principle, additives such as surfactants that affect physical properties of the ink and which have relatively high degrees of freedom of the ink design (Japanese Patent Application Laid-open No. 2000-303014).
Until now, the pigment-based ink and the dye-based ink have not been used simultaneously for a single ink-jet printer because of the difference between the ink physical properties of the pigment-based ink and the dye-based ink. However, in order to effectively utilize the advantages of the respective inks, an ink-jet printer has become commercially available, which carries an ink set for ink-jet recording including a pigment-based ink (black ink) based on the use of a black pigment as a coloring agent and a dye-based ink (color ink) based on the use of a yellow, cyan, or magenta dye as a coloring agent. When letter or character data is printed by using the black pigment-based ink with the ink-jet printer as described above, a letter print, which has excellent visual recognition performance, is obtained, because the printed letters have sharp edges with high contrast as well. On the other hand, for example, when graphic data is printed, the color portion provides vivid colorfulness. When the pigment-based ink is used as the black ink for the ink-jet recording, and the dye-based ink is used as the color ink for the ink-jet recording, then it can be expected that both of the visual recognition performance of the letters and the vividness of the color portion is simultaneously obtained.
However, the following phenomenon occurs in the case of the ink-jet printer which carries a general ink set for ink-jet recording including a pigment-based ink based on the use of a negatively chargeable and self-dispersible pigment and a dye-based ink based on the use of an anionic dye. That is, when the pigment-based ink and the dye-based ink make contact with each other, and they are mixed with each other, then the counter ion such as sodium ion, which is contained in the anionic dye in the dye-based ink, acts on the dispersing group located on the pigment surface of the self-dispersing and negatively chargeable pigment contained in the pigment-based ink. As a result, the degree of dissociation or dissociation ratio is lowered, and the surface charge (charge amount) of the pigment is decreased. Therefore, the electric repulsive force of the pigment is lowered, or the electric repulsive force is lost. The dispersibility of the pigment contained in the pigment-based ink is lowered, and the pigment is aggregated. If the pigment aggregates are formed, the printing head nozzles are clogged in some cases. In other cases, if the pigment aggregates are adhered and secured to the surroundings of the printing head nozzles, the ink-repelling coating surface of the printing head is damaged. Therefore, there is such a high possibility that the discharge failure may be caused, and the printing quality may be deteriorated. According to the observation of the inventors, it has been revealed that the phenomenon as described above is especially conspicuous when a black ink is used as the pigment ink and a magenta ink is used as the dye ink.
The problem of aggregation of the pigment-based ink as described above is conspicuous in an ink-jet printer which is provided with a simple maintenance system in order to reduce the production cost. For example, as described in U.S. Pat. No. 6,631,974 B2 corresponding to Japanese Patent Application Laid-open No. 2002-234151, such an ink-jet printer includes a printing head and a wiper member for collectively wiping the nozzles of respective colors of the printing head. However, the nozzle arrays of two colors are arranged while being extremely close to one another on the printing head. Therefore, it is impossible to divide, one color by one color, the suction cap for sucking and purging the inks in the printing head. In this structure, the nozzle arrays of two colors, which are disposed on the nozzle surface of the printing head, are collectively covered. For example, the nozzle arrays of the black ink and the cyan ink are collectively covered with the suction cap. The inks of two colors, which are simultaneously sucked by the aid of the suction cap, are mixed with each other in the suction cap, and parts of them are adhered to the nozzle surface in some cases. Therefore, when the wiping operation is performed, the mixed ink, which contains the pigment-based ink and the dye-based ink, is adhered, for example, to the wiper member and the nozzle surface of the printing head. If the pigment is aggregated at such positions, various inconveniences arise as described above. In some viewpoints, it may be possible to solve such inconveniences by improving the printer. However, such a countermeasure is extremely disadvantageous in view of the cost.
As described above, when the ink set for ink-jet recording, which is composed of the pigment-based ink and the dye-based ink, is carried on the ink-jet printer provided with the conventional maintenance system which is simple and at the low cost, it is feared that the ink-jet recording cannot be performed at a high quality due to the aggregation of the pigment.